Certain infusion pumps, particularly certain ambulatory insulin infusion pumps that are used to treat people having diabetes, include a spool with a distal section, a collapsible reservoir that contains insulin, vented volume surrounding the collapsible reservoir and a pressure sensor that senses the pressure within the vented volume. The pressure sensor is coupled to a controller or processor that is programmed with an algorithm to control, operate and monitor the infusion pump. The pressure sensor can be used to determine the fluid volume in the collapsible reservoir of the system. In such cases, pressure measurements are taken at intervals and variations in air pressure are used to calculate fluid volume within the collapsible reservoir.
One of the challenges of using the pressure sensor to determine fluid volume in the infusion pump system is that the related calculations are generally made under the assumption that the vented volume air pressure and the fluid pressure in the collapsible reservoir are the same. This assumption is not always true. This is due, for example, to the fact that the collapsible reservoir is not infinitely flexible, especially when it is filled to maximum or near maximum capacity. When the collapsible reservoir is filled to maximum capacity, the envelope of the collapsible reservoir is placed under stress and there can be variations between the pressure inside of the collapsible reservoir and the vented volume surrounding the collapsible reservoir. In the case of the collapsible reservoir being at or near maximum capacity, any compliance in the system, such as, e.g., air bubbles in the fluid or expansion of the fluid-containing structure that does not affect air chamber volume, can cause errors in the fluid volume measurement process as fluid that is removed from the collapsible reservoir in the system will not cause an equal change in air volume of the air in the air chamber.